In the instrumentation, control and related arts, it is often desirable to connect a remote voltage source to a distant point for measurement or control purposes. For example, thermocouples are generally located many feet from a temperature indicating meter. Similarly, other transducers and sensor type voltage sources are often remotely spaced from indicating or control circuitry which is responsive to a sensed condition. In the past, noise considerations have generally necessitated that the voltage source generate a voltage on at least the millivolt level for reliable information transfer with a remote instrument or other receiver.
Previously, low-voltage, low-frequency signals, particularly those in the microvolt to nanovolt range, have been very difficult and often impossible to measure because the signals to be measured were indistinguishable from noise which was both generated within and induced into the system. One major noise source has been found to be a common mode voltage appearing between the "local ground" at the remote low-level voltage source and the "reference ground" at the distant meter, instrument or other receiver. The low-level voltage source is either directly connected to the local ground or effectively connected to the local ground through leakage resistances and leakage capacitances which result from the physical support of the source at local ground as well as the proximity of nearby objects. These values can have an enormously wide range of possibilities, depending in detail on the nature of the source and the specific point of application. Here the source could be a thermocouple, pH electrode or several other common transducers, whereas local ground could be a furnace, machine, or even a human patient. By the nature of any local ground, composed of heavy base metal plates, wires, supporting fixtures, etc., and the fact that rotating machinery, power using equipment and power cables could be in the general vicinity, at least in most industrial or laboratory environments, it is common to find a sizable potential difference between local ground and reference ground. This potential difference, which is predominately steady state 60 Hz. of up to one volt, could under some special cases consist of transients of several hundred volt magnitudes lasting up to a few milliseconds in time.
Where the remote low-level voltage source is connected to a distant meter by a wire, magnetically induced voltages may also occur in the system. This noise source is produced by magnetic flux changes over a period of time. Mechanical motion of charged objects near the low-level voltage source and all along the wire connected to the meter also produce noise transients in the system. The most common form of such "charge sources" are humans, often with at least one meter operator involved. Depending on humidity conditions, a single human being can act typically as a one microfarad capacitor to ground, charged to several thousand volts potential. It is not necessary that humans or other objects actually touch either the wire or immediate fixtures, inasmuch as relative motion causes their distributed capacitance to change sufficiently in time to cause significant transient current to flow in the circuit.
Still another important noise source is galvanomagnetic voltage sources, the principal source of which is the phenomenon of thermoelectricity. These sources are principally D.C. in nature, but can have components of time change representable by thermal time constants associated with the thermal mass and thermal insulation of the connecting wire or circuit involved. Because, in general, the source and meter are at different temperatures, each electrical connection acts as an individual thermocouple wire generating a distributed voltage within itself.
Remote sensing of sub-microvolt level signals at near D.C. frequencies can be seen as being most severely limited by two effects: (1) thermoelectric voltage instability on the signal lines, especially under severe temperature gradient conditions, and (2) inability to remove one end of the source from local ground. These are the main reasons why sub-microvolt level measurements are seldom attempted for remote applications, even with the most advanced state of the art equipment.
One prior art method for increasing the reliability of low-level signal transmission systems involves modulating the low-level D.C. or low-frequency A.C. signal at the common meter input with a chopper circuit. The chopper converts the low-level voltage at the source into an alternating current signal of a higher frequency and of substantially a square wave form. Hitt et al U.S. Pat. No. 3,397,353, issued Aug. 13, 1968, and No. 3,585,518, issued June 15, 1971, teach the use of field effect transistors in a chopper array which is non-symmetrical with respect to common or ground. The chopper utilizes a single balancing capacitor and/or a special transformer to aid in reduction of chopper drive signal to low-level channel coupling. This non-symmetrical array and/or need for a costly balancing transformer limits the complete and economical solution for drive signal coupling problems.
Banasiewicz et al U.S. Pat. No. 3,621,474, issued Nov. 16, 1971, teaches a modulator or chopper using a balanced bridge configuration of field effect transistors (FETs) in obtaining the modulation of a carrier signal with a source signal. This technique of impressing the source signal on the gates of the transistors prevents complete source signal chopping from occurring, which is incompatible with low-level signal modulation, amplification and demodulation measuring techniques.
Lynn et al U.S. Pat. No. 3,612,903, issued Oct. 12, 1971, teaches the use of a balanced bridge field effect transistor chopper circuit of moderate low-level signal capability. The use of junction field effect transistors (JFETs) in the legs of the bridge in a serious limitation, permitting gate drive leakage current to reach the low-level channel. JFETs also require the added use of transformer secondary circuit diodes and discharge resistors that add to the cost and add a potentially troublesome reduction in circuit reliability. The use of series resistors and an adjustable resistor in the low-level channel add cost increases to the chopper and add thermal voltage sources into the low-level channel, a further noise source. Their use of resistor balancing for capacitor coupled gate signal to low-level channel balancing makes the balance condition frequency dependent. Failure to provide heat sinks or other thermal stabilizing influences on this circuitry permits internally generated voltages to limit the Lynn et al chopper to the microvolt or higher voltage ranges.
The above and other prior art chopper circuits have been limited in applications involving very low-level signals due to troublesome noise which generally results from two sources. One is the thermoelectrically caused low-frequency noise resulting from inadequate thermal stabilization of circuit elements and junctions. The other is large common mode signal sources which become incorporated into the signal channel of most prior art chopper circuits by the nature of their non-symmetrical relation to ground. The circuits of the present invention directly address these problems and consequently offer significant improvements over the prior low-level signal processing art.